4 year old having severe stomach pain. Any suggestions?

Hi, I have a 4 year old daughter and every time she is sitting down in the bath tub she complains of her stomach hurting and she has to stand up. It hurts her so bad she is in tears. It doesn't hurt her in the shower or in a pool. I've tried looking up some answers but can't seem to find any. Any opinions will be appreciated. Thank you.